Electret transducer

ABSTRACT

An electret transducer comprising a metallized diaphragm tensioned in juxtaposition with a metallic backplate in which the width to height of the air gap between the two is selected so that the sensitivity remains substantially constant over the life of the transducer. The backplate contains ridges over which the diaphram is tensioned.

United States Patent Madsen et al.

ELECTRET TRANSDUCER inventors: Henning Schmidt Madsen, Hillerod,

Denmark; Cornelis Wilfred Reedyk, Ot-

tawa, Ontario, Canada Assignee: Northern Electric Company Limited,Montreal, Quebec, Canada Filed: Jan. 15, 1971 Appl. N0.: 106,776

U.S. Cl ..179/1l1E ..H04r 19/00 ..179/l 11 E; 307/88 ET Int. Cl Field ofSearch 5] May 16,1972

Primary Examiner-Kathleen H. Claffy Assistant Examiner-Douglas W. OlmsAttorney-John E. Mowle [5 7] ABSTRACT An electret transducer comprisinga metallized diaphragm tensioned in juxtaposition with a metallicbackplate in which the width to height of the air gap between the two isselected so that the sensitivity remains substantially constant over thelife of the transducer. The backplate contains ridges over which thediaphram is tensioned.

7 Claims, 3 Drawing Figures Patented 'May 16, 1972 2 Sheets-Sheet 1INVENTORS HENNING S. MADSEN CORNELIS W. REEDYK BYVZZM g PATENT AGENTPatented May 16, 1972 3,663,768

2 Sheets-Sheet 2 I I0 '5 7/ 3o 0.. '5 |0 Q LU E DIAPHR TENSION NEWTO PERMETER d D:

50 I00 I50 200 EQUIVALENT BIAS VOLTS INVENTORS HENNING S. MADSENCORNELIS W. REEDYK BY %LM :9

PATENT AGENT ELECTRET TRANSDUCER FIELD OF THE INVENTION This inventionrelates to an electret transducer and more particularly to one in whichthe dimensional parameters are selected so that the sensitivity remainssubstantially constant as the surface charge density of the electretdecays.

DESCRIPTION OF THE PRIOR ART Recently, there has been considerabledevelopment of electret transducers which may be utilized as eithermicrophones or loudspeakers. One common form utilizes a metallized thinplastic film or diaphragm which is tensioned in close proximity to aconductive backplate. The film-has a permanent charge on it so that whenan electrical signal is connected across the two conductive surfaces thediaphragm is alternatively attracted to and repelled from the backplateso as to vibrate in the manner of a loudspeaker.

Conversely, when sound pressure waves are applied to the diaphragm anelectrical signal is produced between the two conductive surfaces in amanner similar to that of a condenser microphone. However, unlike thecondenser microphone, no external d-c bias is required. Because the biasin the condenser microphone is supplied from an external source, it canbe readily maintained within close limits. Hence, the sensitivity can beheld within strict limits as a result of which the condenser microphoneis often used as a standard in sound measurements. However, in theelectret microphone, the initial surface charge density on the electretfilm slowly decays over the useful life of the transducer. This isparticularly true when the transducer is subjected to high humidity andtemperature, both of which accelerate the charge decay process.

SUMMARY OF THE INVENTION It has bee discovered that, in an electrettransducer in which the diaphragm is spaced from the backplate by anumber of parallel ridges, by selecting a width between ridges to ridgeheight ratio within a certain range, the sensitivity remainssubstantially constant over a wide range of surface charge densities. Asa result, the sensitivity of the microphone can be held within closetolerances over its useful life.

Thus, in accordance with the present invention there is provided anelectret transducer, which may be either in the form of a microphone ora loudspeaker, comprising a perforated backplate having both aconductive surface and a plurality of parallel ridges on one side. Inaddition, the transducer includes an electret film or diaphragmtensioned over the parallel ridges with a metallized surface on the sideopposite that in contact with the ridges. The ratio of the distancebetween the ridges to the height of the ridges is between about 100 to land 40 to 1 as a result of which the sensitivity remains substantiallyconstant as the surface charge density on the electret diaphragm decaysover its useful life.

The tension on the diaphragm controls the resonant frequency of thetransducer. Thus, a higher tension will produce a higher resonantfrequency. However, a lower tension will produce a greater sensitivityup to the point where the diaphragm collapses. This occurs when adecrease of the air gap, which may be due to an applied positivepressure, causes the electrostatic force due to the charge to increaseby an amount greater than the elastic restoring force that is determinedby the tension on the diaphragm. In Electroacoustics" pp. 179-183, 1954,Harvard University Press, Harvard Monograph in Applied Science Number 5,F .V. Hunt has shown that a theoretical limit of stability is reachedwhen the static air gap, due to a field in the gap, is reduced byone-third that when no field is applied. Thus the electrostatic forcerequired for collapse will depend upon the tension on the diaphragm aswell as other parameters which affect its compliance. In a preferredembodiment, the tension is between about Newtons per meter and Newtonsper meter which provides maximum sensitivity while satisfying the otherparameters mentioned above.

III-1 The sensitivity of the transducer is dependent upon the amount offixed capacity in shunt with the variable capacity. For maximumsensitivity, the former should be kept as small as possible. Thebackplate may be constructed of a plastic material on which is depositeda metallized coating. In order to reduce the fixed capacity of thetransducer in a still more preferred embodiment, only a small portion ofeach of the ridges is metallized in order to interconnect the flatsurface portions of the backplate.

It has been found that utilizing a metallized plastic backplate yieldsan additional advantage in that the coefficient of expansion of thebackplate can be selected to be substantially the same as that of thediaphragm. This maintains a substantially constant tension on the filmwith variations in ambient temperature. It has been achieved in thepresent invention by utilizing metallized acrylonitrile butadienestyrene for the backplate and metallized tetrafluoroethylene for thediaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS -An example embodiment of theinvention will now be described with reference to the accompanyingdrawings in which:

FIG. 1 is an exploded view of an electret transducer in accordance withthe present invention;

FIG. 2 is a cross-sectional view of the assembled electret transducershown in FIG. 1; and

FIG. 3 is a graph of signal output versus equivalent bias voltage forthe electret transducer illustrated in FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, theelectret transducer shown in the form of a microphone comprises aslightly convex backplate 10 having a plurality of parallel ridges 11 onone side thereof. In the present embodiment, the backplate 10 wasconstructed of plastic. In order to provide a conductive surface,portions 12 of the backplate 10 were covered with a metallized coating.Overlying the periphery of the backplate 10 is a thin plastic gasket 13which helps to reduce the stray fixed capacity of the microphone.Stretched over the gasket 13 in contact with the ridges "11 is adiaphragm comprising a thin plastic film 14 having a metallized coating15 on the side opposite that in contact with the ridges. A frame 16 isplaced over the diaphragm 14. The whole assembly is held together by apair of clips 17 and 18 which are snapped over opposed ends of theassembly.

To relieve the back pressure between the diaphragm l4 and the backplate10, a series of holes 19 are located in the backplate 10. It has beenfound that holes of 40 mils diameter on mil centers achieve this withoutexcessively reducing the effective area of the conductive portions 12and hence the variable capacity of the transducer.

An external connection is made to the conductive portions 12 of thebackplate 10 through an electrical connecting portion 20. Electricalconnection to the metallized coating 15 is made by extending thediaphragm 14 around and under the bottom of the clip 18 as shown on theright side of FIG. 2. Hence, the metallized coating 15 contacts the clip18, to which suitable connection can then be made.

While various materials may be used, it has been found that good resultsare obtained if the electret film 14 is made of tetrafluoroethylenewhich is sold under the trademark Teflon. The thickness of the film 14will also affect the performance of the transducer. It has been foundthat very good results can be obtained using I mil Teflon. The thicknessof the metallized coating 15 on the Teflon film 14 is only a smallfraction of the total thickness of the film 14, typically in the orderof 1,000 to 5,000 angstroms. The electret film 14 may be charged usingany one of a number of known charging methods. One such method isdisclosed in Thermal Currents from Corona Charged Mylar by Robert A.Creswell and Martin M. Perlman; Journal of Applied Physics, Vol. 41, No.6, pages 2365- 2375, May 1970. It has been found that the charge on theelectret film 14 has a longer life if the uncoated side of the film 14(that facing the backplate is charged negatively.

While the Teflon film 14 holds a permanent charge very well even inareas of high humidity, it suffers from the disadvantage that whensubject to a tension, it tends to relax or stretch over a prolongedperiod, particularly with expansion and contraction due to variations inambient temperature. This tendency can be reduced by selecting amaterial such as acrylonitrile butadiene styrene for the backplate 10and the frame 16 having similar coefficient of expansion characteristicsto that of Teflon. The use of a plastic for the backplate 10 has thefurther advantage in that only the selected portions 12 contiguous withthe movable areas of the film 14 need be metallized as shown in FIG. 1.This helps to greatly reduce the stray fixed capacity of the transducer,Since the overall sensitivity of the transducer is dependent upon theratio of the movable to fixed capacity of the transducer, the result isincreased sensitivity. The stray capacity can be further reduced asshown in FIG. 1 by metallizing only a small portion 21 of the ridgesinterconnecting the various metallized sections 12.

The sensitivity remains substantially constant, i.e. within a fewdecibels, as the surface charge density on the electret decays over itsuseful life by selecting a width W to height H ratio of between about 40to 1 and 100 to 1, as shown in FIG. 2. However, optimum results appearto be obtained if the ratio of W to H is in the range of 70 to l with aridge height H of about 2 mils, while the transducer is working into atypical load (not shown) such as 6 MO, shunted by pF.

The surface charge density is usually measured by determining theequivalent bias voltage required to neutralize the electrostatic fieldacross the air gap between the plastic film l4 and the backplateportions 12. In a microphone this is measured by connecting a variabled-c source and a load (both not shown) in series with the microphone. Anacoustic signal is then fed to the microphone and the bias is varieduntil the resultant a-c signal across the load is nulled. At this point,the dc voltage from this bias source is read as the equivalent biasvoltage of the surface charge density.

The relationship between the two is given as follows:

0- Ke V /d where:

osurface charge density;

K dielectric constant;

6,, permittivity of free space;

V,, equivalent bias voltage; and

a'= thickness of electret film 14.

A graph of signal output versus equivalent d-c bias for various tensionson the film 14 is shown in FIG. 3 for ratios of W versus H (FlG. 2) of70:1. Evidently, the sensitivity remains substantially flat over theequivalent bias voltage range of from 50 to 200 volts.

It should be noted that the sensitivity may remain substantiallyconstant for equivalent bias voltages greater than 200 volts. However,this is considered to be a practical limit since it is not onlydifficult to charge the film 14 to an equivalent bias voltage greaterthan 200 volts but there is also an increased tendency towards arcingacross the film 14 itself for values beyond this point.

Evidently, the sensitivity is inversely proportioned to the tension onthe electret film 14 while the resonant frequency is nearly proportionalto the square root of the tension. In the preferred embodiment the filmis tensioned in a direction orthogonal to that of the length of theridges. The tension is held between about 10 and 20 Newtons per meter.This yields good sensitivity and reproductibility yet leaves sufficientsafety margin that the film 14 is not in danger ofcollapsing.

What is claimed is:

1. An electret transducer comprising:

a perforated backplate having both a conductive surface and a pluralityofparallel ridges on one side thereof; and an electret diap ragmtensioned over the parallel ridges; the

diaphragm having a metallized surface on the posite that in contact withthe ridges;

in which the ratio of the distance between said ridges to the height ofthe ridges is between about and 40:l whereby the sensitivity of thetransducer remains substantially constant when the equivalent biasvoltage resulting from the surface charge density of the electret variesbetween about 200 and 50 volts.

2. An electret transducer as defined in claim 1 in which the tension onthe diaphragm is between about l0 and 20 New tons per meter.

3. An electret transducer as defined in claim 2 in which the ratio ofthe distance between said ridges to the height of the ridges is about70: l.

4. An electret transducer as defined in claim 1 which has a plurality ofconductive areas between said ridges interconnected by narrow conductiveportions over the ridges so as to minimize the fixed capacity of thetransducer.

5. An electret transducer as defined in claim 1 in which the coefficientof expansion of the perforated backplate and the electret diaphragm aresubstantially the same.

6. An electret transducer as defined in claim 5 in which the material inthe backplate consists of acrylonitrile butadiene styrene and thematerial in the diaphragm consists of tetrafluoroethylene.

7. In an electret transducer comprising:

a perforated backplate having a plurality of parallel ridges on one sidethereof;

an electret diaphragm tensioned over the parallel ridges, the

diaphragm having a metallized surface on the side opposite that incontact with the ridges;

the improvement comprising:

said backplate having conductive areas between said ridgesinterconnected by narrow conductive portions over the ridges so as tominimize the fixed capacity of the transducer.

side op-

1. An electret transducer comprising: a perforated backplate having botha conductive surface and a plurality of parallel ridges on one sidethereof; and an electret diaphragm tensioned over the parallel ridges;the diaphragm having a metallized surface on the side opposite that incontact with the ridges; in which the ratio of the distance between saidridges to the height of the ridges is between about 100:1 and 40:1whereby the sensitivity of the transducer remains substantially constantwhen the equivalent bias voltage resulting from the surface chargedensity of the electret varies between about 200 and 50 volts.
 2. Anelectret transducer as defined in claim 1 in which the tension on thediaphragm is between about 10 and 20 Newtons per meter.
 3. An electrettransducer as defined in claim 2 in which the ratio of the distancebetween said ridges to the height of the ridges is about 70:1.
 4. Anelectret transducer as defined in claim 1 which has a plurality ofconductive areas between said ridges interconnected by narrow conductiveportions over the ridges so as to minimize the fixed capacity of thetransducer.
 5. An electret transducer as defined in claim 1 in which thecoefficient of expansion of the perforated backplate and the electretdiaphragm are substantially the same.
 6. An electret transducer asdefined in claim 5 in which the material in the backplate consists ofacrylonitrile butadiene styrene and the material in the diaphragmconsists of tetrafluoroethylene.
 7. In an electret transducercomprising: a perforated backplate having a plurality of parallel ridgeson one side thereof; an electret diaphragm tensioned over the parallelridges, the diaphragm having a metallized surface on the side oppositethat in contact with the ridges; the improvement comprising: saidbackplate having conductive areas between said ridges interconnected bynarrow conductive portions over the ridges so as to minimize the fixedcapacity of the transducer.